WASHINGTON, Nov. 4 — The battle for Congress rolled into a climactic final weekend with Republican Party leaders saying the best outcome they could foresee was losing 12 seats in the House, but that they were increasingly resigned to losing at least 15 and therefore control of the House to Democrats for the first time in 12 years.

Democrats and Republicans said the battle over the Senate had grown fluid going into the final hours before the election Tuesday. Democrats said they thought they were almost certain to gain four or five seats and still had a shot at the six they need to take control. Republicans were pouring money into Senate races in Michigan and Maryland this weekend to take advantage of what they described as last-minute opportunities, however slight, in states currently held by Democrats.

Party strategists on both sides, speaking in interviews after they had finished doing their last polls and making their final purchases of television time, said they were running advertisements in more than 50 Congressional districts this weekend, far more than anyone thought would be in play at this stage.

Nearly all of those seats are held by Republicans, underscoring the degree to which President Bush and his party have been forced onto the defensive two years after Mr. Bush claimed that his re-election had given him the political capital to carry out an ambitious domestic and foreign agenda.

As the final weekend began, the two parties made their final tactical moves as their candidates sparred over the war, the economy, corruption and competence and elaborate get-out-the-vote efforts campaigns were rolled out. At stake was not just control of the House and Senate, but potentially the course of the Bush presidency in its last two years and in particular the debate over how to proceed in Iraq.

Democrats bought advertising time in yet another House race that had long been considered safe for Republicans, that of Representative Marilyn Musgrave of Colorado. Mr. Bush appeared at a rally in Ms. Musgrave’s district on Saturday morning, part of a late flurry of campaigning by the president aimed at shoring up struggling Republicans in some of the reddest states in the country, including Nebraska and Kansas. In another bit of news that sent a chill through many Republicans, a University of New Hampshire Poll showed Representative Charles. Bass, a popular moderate Republican who had not been seen as vulnerable this year, trailing his opponent.

“It’s the worst political environment for Republican candidates since Watergate,” said Glen Bolger, a Republican pollster working in many of the top races this year.

Joe Gaylord, who was the political lieutenant to Newt Gingrich when Mr. Gingrich led the Republican takeover of the House in 1994, said that based on polling he had seen in recent weeks, he expected his party to lose from 25 seats to 30 seats Tuesday. That general assessment was repeatedly echoed in interviews by Republicans close to the White House and the Republican National Committee.

“It’s very grim,” Mr. Gaylord said. “Things are dreadful out there.”

Representative Thomas M. Davis III, Republican of Virginia and a veteran party strategist, noted a significan number of races remained very close, adding: “There’s no question we’re going to take a hit. The only question is how hard it would be.”

Still, some Republicans, and some Democrats, said Republicans could be bolstered by structural advantages that could at the very least minimize the party’s losses. Aides to both parties said that at least 20 races were close enough that struggling Republican incumbents could be pulled to victory by the party’s sophisticated get-out-the-vote operation. Karl Rove, Mr. Bush’s chief political adviser, has assured nervous associates that the Republican turnout operation would help save the party from electoral disaster.

“There are a lot of seats on the bubble and that is why turnout makes such a difference,” said Representative Eric Cantor of Virginia, the deputy Republican whip. Representative Thomas M. Reynolds, the New York Republican heading his party’s effort to hold the House, said: “Turnout will be key to us in these three dozen races that are close across the country.” In a sign of how unexpectedly challenging the climate is for Republicans, Mr. Reynolds has had to devote much of his time to his own tough re-election battle.

I can't predict numbers, but I believe they are going to lose seats they think are safe. Someone is going to sneak in and take a seat the GOP didn't worry about.

If the GOP loses, it will be for one simple reason, the scandals shrank the base, and no matter what his short term tactics have done in the past, they aren't designed for this kind of resentment. The key to Rove's success is getting the base out. That can't help if the base no longer trusts the GOP to do the right thing.

Look, things may change, but if this is the public spin, they could be facing losses double that.

Remember, these are comments for attribution. And this is not the kind of thing to fire people up to vote Republican. It is, however, encouraging to Dems. And when you start saying you're gonna lose big numbers, what aren't you saying?